The present invention relates generally to diffusion coating methods and specifically to a new and improved method for aluminizing steel components, and especially boiler components, to improve resistance to high-temperature corrosion.
Aluminum diffusion coating has been widely used for decades to protect various components from high-temperature corrosion attack. By way of example and not limitation, the aerospace industry has been applying aluminum diffusion coating on the surfaces of turbine blades to prolong the service lives of gas engines. Accordingly, several prior art aluminizing processes for the production of aluminum diffusion coating on steels have been developed and used on large components, such as furnace wall panels for boilers, in order to improve the quality of the component and/or to improve the process control involved in producing the component.
One aluminizing method is described in U.S. Pat. No. 5,135,777 to Davis, et al., which is hereby incorporated by reference. Essentially, this method involves placing a slurry-coated ceramic alumino-silicate fiber next to a workpiece and heating the combination until the slurry coating diffuses onto the workpiece. Significantly, a halide activator must be included in the slurry coating in order to effect the diffusion of the slurry material.
Another aluminizing process known to those skilled in the art involves applying a layer of commercial-grade aluminum onto the surfaces of a workpiece by means of thermal spray (e.g., plasma or arc spray). In aluminum thermal spray, feed material in the form of powder or wire is rapidly melted and injected to the substrate. The molten aluminum particles spread out and splatter as they strike the surfaces to be coated. These particles first bond to the substrate and then to each other, forming a surface layer. The aluminum sprayed parts are then heat treated at elevated temperatures in a furnace under an inert or reducing atmosphere. Such heating causes the aluminum to diffuse from the sprayed layer into the substrate surfaces of the workpiece. Once this diffusion occurs, the aluminum becomes an integral part of the workpiece and any remnant of the aluminum spray layer can be easily removed, leaving only an aluminizing diffusion coating on the workpiece. Although no halide activator is utilized in this process, this process has been limited solely to the use of a single element (i.e., commercial-grade aluminum), rather than a combination of elements. Further, as demonstrated by U.S. Pat. No. 5,873,951 to Wynns, et al. (which is hereby incorporated in its entirety), those skilled in the art had believed that introduction of chromium into an aluminizing process will produce instability in the alloy structures. Further, as discussed in Wynns, et al., many prior art methods (including Wynns, et al.) contemplated multi-step processes for diffusing aluminum and, in some cases, chromium or silicon.
Moreover, when the workpiece consists of steel, use of a thermal spray aluminizing process produces a multi-layered coating structure on the steel surface. The outer layers of this multi-layer coating structure consist of Fe--Al ordered phases, also known as intermetallic compounds, such as FeAl and Fe.sub.3 Al. Although these aluminides are very corrosion resistant, they possess very low fracture toughness which makes them brittle and susceptible to mechanical damage. As a result, a workpiece aluminized by the thermal spray process must be handled with care to avoid accidental cracking and spallation of the coating.
In light of the foregoing, a diffusion coating material and method with improved fracture toughness is needed. Further, a thermal spray material for aluminizing which would allow multiple elements to be diffused simultaneously into steel surfaces would be welcome by the industry. Finally, a method for simultaneously introducing aluminum in conjunction with minor amounts boron and/or chromium into steel surfaces in order to increase fracture toughness without the use of a halide activator is desired.